1. Field of the Invention
This invention relates to equipment and processes for thermally processing food products, and more particularly concerns apparatuses and processes for thermally processing food products hermetically sealed in plastic containers, especially thin-walled, high-profile plastic containers.
2. Description of the Prior Art
The thermal processing of sealed, food-filled, thin-walled, high-profile plastic containers presents some unique problems not previously encountered with rigid metal containers. Since most plastic materials currently approved for contact with food tend to lose their structural definition (shape) as temperatures increase above 170 degrees Fahrenheit, maintaining a balance between the internal pressure inside the plastic container and the external pressure on the plastic container inside the sterilizer during the sterilization process is very critical for maintaining the shape of the container and for insuring that the seal remains intact. If pressure imbalance between the internal pressure inside the plastic container and the external pressure on the outside of the plastic container become too great during thermal processing, the container bulges and/or panels. This is a major problem in the industry, since a bulged or panelled container is not sold, but rather is rejected by the food packer and thrown away.
One method of producing acceptable food-filled plastic containers with good seals is to constantly monitor for pressure differences between the pressure inside the plastic container and the pressure outside the plastic container in the processing vessel which occur during thermal processing, and then to correspondingly adjust the processing controls to balance these pressures.
Such a method of balancing the pressures inside and outside the plastic container require: (a) different pressure profiles for different food materials, since different foods usually have different thermal expansion properties; and (b) longer ramping pressure profiles during the heating cycle and the cooling cycle of the sterilization process, which significantly reduces the production capacity (throughput) of the processing unit during production.
This method of thermally processing foods packed and sealed in plastic containers by balancing pressures inside and outside the sealed, food-filled plastic container is currently used with varying degrees of success by food processors, and the best results using this method are achieved with low-profile plastic trays and thick-walled plastic cups which, for the most part, are still being closed with a metal lid using the conventional double-seam seal.
Under the known technology, cup-like plastic containers that are closed either by conduction or induction sealing using a thin foil lidding material require overpressure during the sterilization process not only to preserve the container shape but to prevent the lid from bursting at the seal track. Since the foil lid does not use a double-seam seal, it is very susceptible to rupturing if the internal pressure inside the container is not compensated for during thermal processing.
Since the amount of air in the sealed, food-filled plastic container has an effect on the internal pressure inside the plastic container, a foil or plastic lid material which, is either preformed or shaped during the closing step, is used to eliminate any headspace between the product and the lid. Forming the lid in this manner creates a condition in the package known as a "hydropack" which allows the internal pressure to be accurately calculated during the sterilization cycle. The drawback with this method is that it requires the use of expensive lidding material and slower sealing speeds.
Further, since fill weights tend to vary to some degree during the filling of food products into the plastic containers, the amount of headspace in the container and, correspondingly, the amount of internal pressure created in the container during thermal processing varies from container to container. Thus, the pressure profile used for a particular thermal processing cycle does not compensate for all the variations in fill weights and headspace of each container being processed, and therefore a certain percentage of the containers are distorted (e.g., bulged or panelled walls or ruptured seals) during the thermal processing.
Material redistribution and shrinkage may also occur during thermal processing at elevated temperatures, particularly plastic containers having a plurality of layers in its walls. Further, if not controlled, rippling of the walls of the plastic container may occur during thermal processing at elevated temperatures, which also causes package distortion.
Moreover, the known technology for thermally processing sealed, food-filled plastic containers is very complex, relying on sophisticated process controls and equipment for balancing pressure inside the plastic container with the pressure on the plastic container outside the plastic container to preserve the seal and to limit distortion of the plastic container. Further, the known technology does not effectively thermally process sealed, food-filled, thin-walled, high-profile plastic containers, since thermal processing of such containers using the known technology usually results in some distortion of the containers.
In summary, there are a multitude of problems that are associated with the thermal processing of sealed, food-filled plastic containers at elevated temperatures, and these problems include:
flexibility of plastic at elevated temperatures; PA0 elaborate and long processing cycles; PA0 expensive lidding material; PA0 slow sealing operations with preformed lids; PA0 distorted containers due to pressure imbalances; PA0 material redistribution and shrinkage; PA0 material cost for plastic; PA0 limited selection of thermal processing equipment.